Assorted Vehicle Design Rules

Using Exact Barrel Lengths

A quick way to determine the volume of real-world vehicles


For the following calculations, replace the values in GURPS Vehicles, 2nd Edition for barrel length with the following formulas (in all cases, Length is the barrel length in calibers, ie. 70 for a 75mm/L70 cannon). See sidebar, p.VE99 for more on barrel lengths. Note that this is only for gunpowder weapons.

Damage: (Length * 0.00824 + 0.3145)
1/2 Damage Range: (Length * 0.00963 + 0.7694)
Weight: (Length * 0.01096 + 0.0062)

The numbers work well for Long or longer barrels, but tends to break down badly for Very Short and Extremely Short barrels. Medium and Short barrels give about 10% more range and weight and 10% less damage with these formula. There is also some appreciable error for larger bore weapons (roughly 155mm and up) for all three stats. However, much of this falls within the 20% "fudge factor" mentioned in the sidebar on p.VE97, Real World Weapons.

The intent of these formulas is to (a) allow for a better fine tuning of slugthrowers and (b) make designing weapons with a spreadsheet easier where you have to look up or memorize fewer numbers (okay, you could use HLOOKUP, VLOOPKUP or similar fuctions, but what if you copy the weapon data to a vehicle's spreadsheet? -- major trouble).

I still haven't figured out what to do yet for ammo weight.


These formulas allow a GM to quickly determine a reasonable volume for real world vehicles. He can then look up the surface area and, by estimating frame strength, come up with somewhat reasonable Hit Point totals (which are required for even Basic vehicle combat).

('^2' means to square the previous number)

Tracked Vehicles
Body Volume: ((Length * 1.4)^2)/1.6
Track Volume: ((Length * 1.4)^2)/2.67 for all tracks
Turret Volume: For post-WWII tanks, use 1/2 body volume. For WWII and non-tank AFVs, use 1/3 volume (1/4 on some)

Use this volume to look up surface area on p.VE18.

Wheeled Vehicles
Body Volume: ((Length * 1.1)^2)/X (x=1.2 if off-road, 1.1 if normal)
Wheel Volume: ((Length * 1.1)^2)/X (x=6 if off-road, 11 if normal) for all wheels
Turret: Use 10% body volume for small turrets, 25% for medium turrets and 35% for large turrets

Again, look up the surface area on p.VE18.

Aircraft
Body Volume: (Length/2)^2
Wing Volume: divide the wing's surface area by 2 to find the surface area per wing. If not given, use 10% of the body's volume for each wing. Decide what type (p.VE8, 18) of wing the plane has.

Again, look up the surface area on p.VE18.

Frame Strength
Use the guide on p.VE18 to determine frame strength for the vehicle. Determine Hit Points normally. 


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